RFC 7787

Distributed Node Consensus Protocol

Internet Engineering Task Force (IETF) M. Stenberg
Request for Comments: 7787 S. Barth
Category: Standards Track Independent
ISSN: 2070-1721 April 2016 Distributed Node Consensus Protocol
Abstract
This document describes the Distributed Node Consensus Protocol
(DNCP), a generic state synchronization protocol that uses the
Trickle algorithm and hash trees. DNCP is an abstract protocol and
must be combined with a specific profile to make a complete
implementable protocol.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7787.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1.1. Applicability
DNCP is useful for cases like autonomous bootstrapping, discovery,
and negotiation of embedded network devices like routers.
Furthermore, it can be used as a basis to run distributed algorithms
like [RFC7596] or use cases as described in Appendix C. DNCP is
abstract, which allows it to be tuned to a variety of applications by
defining profiles. These profiles include choices of:
- unicast transport: a datagram or stream-oriented protocol (e.g.,
TCP, UDP, or the Stream Control Transmission Protocol (SCTP)) for
generic protocol operation.
- optional transport security: whether and when to use security
based on Transport Layer Security (TLS) or Datagram Transport
Layer Security (DTLS), if supported over the chosen transport.
- optional multicast transport: a multicast-capable protocol like
UDP allowing autonomous peer discovery or more efficient use of
multiple access links.
- communication scopes: using either hop by hop only relying on
link-local addressing (e.g., for LANs), addresses with broader
scopes (e.g., over WANs or the Internet) relying on an existing
routing infrastructure, or a combination of both (e.g., to
exchange state between multiple LANs over a WAN or the Internet).
- payloads: additional specific payloads (e.g., IANA standardized,
enterprise-specific, or private use).
- extensions: possible protocol extensions, either as predefined in
this document or specific for a particular use case.
However, there are certain cases where the protocol as defined in
this document is a less suitable choice. This list provides an
overview while the following paragraphs provide more detailed
guidance on the individual matters.
- large amounts of data: nodes are limited to 64 KB of published
data.
- very dense unicast-only networks: nodes include information about
all immediate neighbors as part of their published data.
- predominantly minimal data changes: node data is always
transported as is, leading to a relatively large transmission
overhead for changes affecting only a small part of it.

- frequently changing data: DNCP with its use of Trickle is
optimized for the steady state and less efficient otherwise.
- large amounts of very constrained nodes: DNCP requires each node
to store the entirety of the data published by all nodes.
The topology of the devices is not limited and automatically
discovered. When relying on link-local communication exclusively,
all links having DNCP nodes need to be at least transitively
connected by routers running the protocol on multiple endpoints in
order to form a connected network. However, there is no requirement
for every device in a physical network to run the protocol.
Especially if globally scoped addresses are used, DNCP peers do not
need to be on the same or even neighboring physical links.
Autonomous discovery features are usually used in local network
scenarios; however, with security enabled, DNCP can also be used over
unsecured public networks. Network size is restricted merely by the
capabilities of the devices, i.e., each DNCP node needs to be able to
store the entirety of the data published by all nodes. The data
associated with each individual node identifier is limited to about
64 KB in this document; however, protocol extensions could be defined
to mitigate this or other protocol limitations if the need arises.
DNCP is most suitable for data that changes only infrequently to gain
the maximum benefit from using Trickle. As the network of nodes
grows, or the frequency of data changes per node increases, Trickle
is eventually used less and less, and the benefit of using DNCP
diminishes. In these cases, Trickle just provides extra complexity
within the specification and little added value.
The suitability of DNCP for a particular application can be roughly
evaluated by considering the expected average network-wide state
change interval A_NC_I; it is computed by dividing the mean interval
at which a node originates a new TLV set by the number of
participating nodes. If keep-alives are used, A_NC_I is the minimum
of the computed A_NC_I and the keep-alive interval. If A_NC_I is
less than the (application-specific) Trickle minimum interval, DNCP
is most likely unsuitable for the application as Trickle will not be
utilized most of the time.
If constant rapid state changes are needed, the preferable choice is
to use an additional point-to-point channel whose address or locator
is published using DNCP. Nevertheless, if doing so does not raise
A_NC_I above the (sensibly chosen) Trickle interval parameters for a
particular application, using DNCP is probably not suitable for the
application.

Another consideration is the size of the published TLV set by a node
compared to the size of deltas in the TLV set. If the TLV set
published by a node is very large, and has frequent small changes,
DNCP as currently specified in this specification may be unsuitable
as it lacks a delta synchronization scheme to keep implementation
simple.
DNCP can be used in networks where only unicast transport is
available. While DNCP uses the least amount of bandwidth when
multicast is utilized, even in pure unicast mode, the use of Trickle
(ideally with k < 2) results in a protocol with an exponential
backoff timer and fewer transmissions than a simpler protocol not
using Trickle.
2. Terminology
DNCP profile the values for the set of parameters given in
Section 9. They are prefixed with DNCP_ in this
document. The profile also specifies the set of
optional DNCP extensions to be used. For a simple
example DNCP profile, see Appendix C.
DNCP-based a protocol that provides a DNCP profile, according
protocol to Section 9, and zero or more TLV assignments from
the per-DNCP profile TLV registry as well as their
processing rules.
DNCP node a single node that runs a DNCP-based protocol.
Link a link-layer media over which directly connected
nodes can communicate.
DNCP network a set of DNCP nodes running a DNCP-based
protocol(s) with a matching DNCP profile(s). The
set consists of nodes that have discovered each
other using the transport method defined in the
DNCP profile, via multicast on local links, and/or
by using unicast communication.
Node identifier an opaque fixed-length identifier consisting of
DNCP_NODE_IDENTIFIER_LENGTH bytes that uniquely
identifies a DNCP node within a DNCP network.
Interface a node's attachment to a particular link.
Address an identifier used as the source or destination of
a DNCP message flow, e.g., a tuple (IPv6 address,
UDP port) for an IPv6 UDP transport.

Endpoint a locally configured termination point for
(potential or established) DNCP message flows. An
endpoint is the source and destination for separate
unicast message flows to individual nodes and
optionally for multicast messages to all thereby
reachable nodes (e.g., for node discovery).
Endpoints are usually in one of the transport modes
specified in Section 4.2.
Endpoint a 32-bit opaque and locally unique value, which
identifier identifies a particular endpoint of a particular
DNCP node. The value 0 is reserved for DNCP and
DNCP-based protocol purposes and not used to
identify an actual endpoint. This definition is in
sync with the interface index definition in
[RFC3493], as the non-zero small positive integers
should comfortably fit within 32 bits.
Peer another DNCP node with which a DNCP node
communicates using at least one particular local
and remote endpoint pair.
Node data a set of TLVs published and owned by a node in the
DNCP network. Other nodes pass it along as is,
even if they cannot fully interpret it.
Origination time the (estimated) time when the node data set with
the current sequence number was published.
Node state a set of metadata attributes for node data. It
includes a sequence number for versioning, a hash
value for comparing equality of stored node data,
and a timestamp indicating the time passed since
its last publication (i.e., since the origination
time). The hash function and the length of the
hash value are defined in the DNCP profile.
Network state a hash value that represents the current state of
hash the network. The hash function and the length of
the hash value are defined in the DNCP profile.
Whenever a node is added, removed, or updates its
published node data, this hash value changes as
well. For calculation, please see Section 4.1.
Trust verdict a statement about the trustworthiness of a
certificate announced by a node participating in
the certificate-based trust consensus mechanism.

Effective trust the trust verdict with the highest priority within
verdict the set of trust verdicts announced for the
certificate in the DNCP network.
Topology graph the undirected graph of DNCP nodes produced by
retaining only bidirectional peer relationships
between nodes.
Bidirectionally a peer is locally unidirectionally reachable if a
reachable consistent multicast or any unicast DNCP message
has been received by the local node (see Section
4.5). If said peer in return also considers the
local node unidirectionally reachable, then
bidirectionally reachability is established. As
this process is based on publishing peer
relationships and evaluating the resulting topology
graph as described in Section 4.6, this information
is available to the whole DNCP network.
Trickle instance a distinct Trickle [RFC6206] algorithm state kept
by a node (Section 5) and related to an endpoint or
a particular (peer, endpoint) tuple with Trickle
variables I, t, and c. See Section 4.3.
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
3. Overview
DNCP operates primarily using unicast exchanges between nodes, and it
may use multicast for Trickle-based shared state dissemination and
topology discovery. If used in pure unicast mode with unreliable
transport, Trickle is also used between peers.
DNCP is based on exchanging TLVs (Section 7) and defines a set of
mandatory and optional ones for its operation. They are categorized
into TLVs for requesting information (Section 7.1), transmitting data
(Section 7.2), and being published as data (Section 7.3). DNCP-based
protocols usually specify additional ones to extend the capabilities.
DNCP discovers the topology of the nodes in the DNCP network and
maintains the liveliness of published node data by ensuring that the
publishing node is bidirectionally reachable. New potential peers
can be discovered autonomously on multicast-enabled links; their

addresses may be manually configured or they may be found by some
other means defined in the particular DNCP profile. The DNCP profile
may specify, for example, a well-known anycast address or provision
the remote address to contact via some other protocol such as DHCPv6
[RFC3315].
A hash tree of height 1, rooted in itself, is maintained by each node
to represent the state of all currently reachable nodes (see
Section 4.1), and the Trickle algorithm is used to trigger
synchronization (see Section 4.3). The need to check peer nodes for
state changes is thereby determined by comparing the current root of
their respective hash trees, i.e., their individually calculated
network state hashes.
Before joining a DNCP network, a node starts with a hash tree that
has only one leaf if the node publishes some TLVs, and no leaves
otherwise. It then announces the network state hash calculated from
the hash tree by means of the Trickle algorithm on all its configured
endpoints.
When an update is detected by a node (e.g., by receiving a different
network state hash from a peer), the originator of the event is
requested to provide a list of the state of all nodes, i.e., all the
information it uses to calculate its own hash tree. The node uses
the list to determine whether its own information is outdated and --
if necessary -- requests the actual node data that has changed.
Whenever a node's local copy of any node data and its hash tree are
updated (e.g., due to its own or another node's node state changing
or due to a peer being added or removed), its Trickle instances are
reset, which eventually causes any update to be propagated to all of
its peers.
4. Operation
4.1. Hash Tree
Each DNCP node maintains an arbitrary width hash tree of height 1.
The root of the tree represents the overall network state hash and is
used to determine whether the view of the network of two or more
nodes is consistent and shared. Each leaf represents one
bidirectionally reachable DNCP node. Every time a node is added or
removed from the topology graph (Section 4.6), it is likewise added
or removed as a leaf. At any time, the leaves of the tree are
ordered in ascending order of the node identifiers of the nodes they
represent.

4.1.1. Calculating Network State and Node Data Hashes
The network state hash and the node data hashes are calculated using
the hash function defined in the DNCP profile (Section 9) and
truncated to the number of bits specified therein.
Individual node data hashes are calculated by applying the function
and truncation on the respective node's node data as published in the
Node State TLV. Such node data sets are always ordered as defined in
Section 7.2.3.
The network state hash is calculated by applying the function and
truncation on the concatenated network state. This state is formed
by first concatenating each node's sequence number (in network byte
order) with its node data hash to form a per-node datum for each
node. These per-node data are then concatenated in ascending order
of the respective node's node identifier, i.e., in the order that the
nodes appear in the hash tree.
4.1.2. Updating Network State and Node Data Hashes
The network state hash and the node data hashes are updated on-demand
and whenever any locally stored per-node state changes. This
includes local unidirectional reachability encoded in the published
Peer TLVs (Section 7.3.1) and -- when combined with remote data --
results in awareness of bidirectional reachability changes.
4.2. Data Transport
DNCP has few requirements for the underlying transport; it requires
some way of transmitting either a unicast datagram or stream data to
a peer and, if used in multicast mode, a way of sending multicast
datagrams. As multicast is used only to identify potential new DNCP
nodes and to send status messages that merely notify that a unicast
exchange should be triggered, the multicast transport does not have
to be secured. If unicast security is desired and one of the
built-in security methods is to be used, support for some TLS-derived
transport scheme -- such as TLS [RFC5246] on top of TCP or DTLS
[RFC6347] on top of UDP -- is also required. They provide for
integrity protection and confidentiality of the node data, as well as
authentication and authorization using the schemes defined in
"Security and Trust Management" (Section 8). A specific definition
of the transport(s) in use and its parameters MUST be provided by the
DNCP profile.
TLVs (Section 7) are sent across the transport as is, and they SHOULD
be sent together where, e.g., MTU considerations do not recommend
sending them in multiple batches. DNCP does not fragment or

reassemble TLVs; thus, it MUST be ensured that the underlying
transport performs these operations should they be necessary. If
this document indicates sending one or more TLVs, then the sending
node does not need to keep track of the packets sent after handing
them over to the respective transport, i.e., reliable DNCP operation
is ensured merely by the explicitly defined timers and state machines
such as Trickle (Section 4.3). TLVs in general are handled
individually and statelessly (and thus do not need to be sent in any
particular order) with one exception: To form bidirectional peer
relationships, DNCP requires identification of the endpoints used for
communication. As bidirectional peer relationships are required for
validating liveliness of published node data as described in
Section 4.6, a DNCP node MUST send a Node Endpoint TLV
(Section 7.2.1). When it is sent varies, depending on the underlying
transport, but conceptually it should be available whenever
processing a Network State TLV:
o If using a stream transport, the TLV MUST be sent at least once
per connection but SHOULD NOT be sent more than once.
o If using a datagram transport, it MUST be included in every
datagram that also contains a Network State TLV (Section 7.2.2)
and MUST be located before any such TLV. It SHOULD also be
included in any other datagram to speed up initial peer detection.
Given the assorted transport options as well as potential endpoint
configuration, a DNCP endpoint may be used in various transport
modes:
Unicast:
* If only reliable unicast transport is used, Trickle is not used
at all. Whenever the locally calculated network state hash
changes, a single Network State TLV (Section 7.2.2) is sent to
every unicast peer. Additionally, recently changed Node State
TLVs (Section 7.2.3) MAY be included.
* If only unreliable unicast transport is used, Trickle state is
kept per peer, and it is used to send Network State TLVs
intermittently, as specified in Section 4.3.
Multicast+Unicast: If multicast datagram transport is available on
an endpoint, Trickle state is only maintained for the endpoint as
a whole. It is used to send Network State TLVs periodically, as
specified in Section 4.3. Additionally, per-endpoint keep-alives
MAY be defined in the DNCP profile, as specified in Section 6.1.2.

MulticastListen+Unicast: Just like unicast, except multicast
transmissions are listened to in order to detect changes of the
highest node identifier. This mode is used only if the DNCP
profile supports dense multicast-enabled link optimization
(Section 6.2).
4.3. Trickle-Driven Status Updates
The Trickle algorithm [RFC6206] is used to ensure protocol
reliability over unreliable multicast or unicast transports. For
reliable unicast transports, its actual algorithm is unnecessary and
omitted (Section 4.2). DNCP maintains multiple Trickle states as
defined in Section 5. Each such state can be based on different
parameters (see below) and is responsible for ensuring that a
specific peer or all peers on the respective endpoint are regularly
provided with the node's current locally calculated network state
hash for state comparison, i.e., to detect potential divergence in
the perceived network state.
Trickle defines 3 parameters: Imin, Imax, and k. Imin and Imax
represent the minimum value for I and the maximum number of doublings
of Imin, where I is the time interval during which at least k Trickle
updates must be seen on an endpoint to prevent local state
transmission. The actual suggested Trickle algorithm parameters are
DNCP profile specific, as described in Section 9.
The Trickle state for all Trickle instances defined in Section 5 is
considered inconsistent and reset if and only if the locally
calculated network state hash changes. This occurs either due to a
change in the local node's own node data or due to the receipt of
more recent data from another node as explained in Section 4.1. A
node MUST NOT reset its Trickle state merely based on receiving a
Network State TLV (Section 7.2.2) with a network state hash that is
different from its locally calculated one.
Every time a particular Trickle instance indicates that an update
should be sent, the node MUST send a Network State TLV
(Section 7.2.2) if and only if:
o the endpoint is in Multicast+Unicast transport mode, in which case
the TLV MUST be sent over multicast.
o the endpoint is NOT in Multicast+Unicast transport mode, and the
unicast transport is unreliable, in which case the TLV MUST be
sent over unicast.

A (sub)set of all Node State TLVs (Section 7.2.3) MAY also be
included, unless it is defined as undesirable for some reason by the
DNCP profile or to avoid exposure of the node state TLVs by
transmitting them within insecure multicast when using secure
unicast.
4.4. Processing of Received TLVs
This section describes how received TLVs are processed. The DNCP
profile may specify when to ignore particular TLVs, e.g., to modify
security properties -- see Section 9 for what may be safely defined
to be ignored in a profile. Any 'reply' mentioned in the steps below
denotes the sending of the specified TLV(s) to the originator of the
TLV being processed. All such replies MUST be sent using unicast.
If the TLV being replied to was received via multicast and it was
sent to a multiple access link, the reply MUST be delayed by a random
time span in [0, Imin/2], to avoid potential simultaneous replies
that may cause problems on some links, unless specified differently
in the DNCP profile. The sending of replies MAY also be rate limited
or omitted for a short period of time by an implementation. However,
if the TLV is not forbidden by the DNCP profile, an implementation
MUST reply to retransmissions of the TLV with a non-zero probability
to avoid starvation, which would break the state synchronization.
A DNCP node MUST process TLVs received from any valid (e.g.,
correctly scoped) address, as specified by the DNCP profile and the
configuration of a particular endpoint, whether this address is known
to be the address of a peer or not. This provision satisfies the
needs of monitoring or other host software that needs to discover the
DNCP topology without adding to the state in the network.
Upon receipt of:
o Request Network State TLV (Section 7.1.1): The receiver MUST reply
with a Network State TLV (Section 7.2.2) and a Node State TLV
(Section 7.2.3) for each node data used to calculate the network
state hash. The Node State TLVs SHOULD NOT contain the optional
node data part to avoid redundant transmission of node data,
unless explicitly specified in the DNCP profile.
o Request Node State TLV (Section 7.1.2): If the receiver has node
data for the corresponding node, it MUST reply with a Node State
TLV (Section 7.2.3) for the corresponding node. The optional node
data part MUST be included in the TLV.
o Network State TLV (Section 7.2.2): If the network state hash
differs from the locally calculated network state hash, and the
receiver is unaware of any particular node state differences with

the sender, the receiver MUST reply with a Request Network State
TLV (Section 7.1.1). These replies MUST be rate limited to only
at most one reply per link per unique network state hash within
Imin. The simplest way to ensure this rate limit is a timestamp
indicating requests and sending at most one Request Network State
TLV (Section 7.1.1) per Imin. To facilitate faster state
synchronization, if a Request Network State TLV is sent in a
reply, a local, current Network State TLV MAY also be sent.
o Node State TLV (Section 7.2.3):
* If the node identifier matches the local node identifier and
the TLV has a greater sequence number than its current local
value, or the same sequence number and a different hash, the
node SHOULD republish its own node data with a sequence number
significantly greater than the received one (e.g., 1000) to
reclaim the node identifier. This difference is needed in
order to ensure that it is higher than any potentially
lingering copies of the node state in the network. This may
occur normally once due to the local node restarting and not
storing the most recently used sequence number. If this occurs
more than once or for nodes not republishing their own node
data, the DNCP profile MUST provide guidance on how to handle
these situations as it indicates the existence of another
active node with the same node identifier.
* If the node identifier does not match the local node
identifier, and one or more of the following conditions are
true:
+ The local information is outdated for the corresponding node
(the local sequence number is less than that within the
TLV).
+ The local information is potentially incorrect (the local
sequence number matches but the node data hash differs).
+ There is no data for that node altogether.
Then:
+ If the TLV contains the Node Data field, it SHOULD also be
verified by ensuring that the locally calculated hash of the
node data matches the content of the H(Node Data) field
within the TLV. If they differ, the TLV SHOULD be ignored
and not processed further.

+ If the TLV does not contain the Node Data field, and the
H(Node Data) field within the TLV differs from the local
node data hash for that node (or there is none), the
receiver MUST reply with a Request Node State TLV
(Section 7.1.2) for the corresponding node.
+ Otherwise, the receiver MUST update its locally stored state
for that node (node data based on the Node Data field if
present, sequence number, and relative time) to match the
received TLV.
For comparison purposes of the sequence number, a looping
comparison function MUST be used to avoid problems in case of
overflow. The comparison function a < b <=> ((a - b) % (2^32)) &
(2^31) != 0 where (a % b) represents the remainder of a modulo b
and (a & b) represents bitwise conjunction of a and b is
RECOMMENDED unless the DNCP profile defines another.
o Any other TLV: TLVs not recognized by the receiver MUST be
silently ignored unless they are sent within another TLV (for
example, TLVs within the Node Data field of a Node State TLV).
TLVs within the Node Data field of the Node State TLV not
recognized by the receiver MUST be retained for distribution to
other nodes and for calculation of the node data hash as described
in Section 7.2.3 but are ignored for other purposes.
If secure unicast transport is configured for an endpoint, any Node
State TLVs received over insecure multicast MUST be silently ignored.
4.5. Discovering, Adding, and Removing Peers
Peer relations are established between neighbors using one or more
mutually connected endpoints. Such neighbors exchange information
about network state and published data directly, and through
transitivity, this information then propagates throughout the
network.
New peers are discovered using the regular unicast or multicast
transport defined in the DNCP profile (Section 9). This process is
not distinguished from peer addition, i.e., an unknown peer is simply
discovered by receiving regular DNCP protocol TLVs from it, and
dedicated discovery messages or TLVs do not exist. For unicast-only
transports, the individual node's transport addresses are
preconfigured or obtained using an external service discovery
protocol. In the presence of a multicast transport, messages from
unknown peers are handled in the same way as multicast messages from
peers that are already known; thus, new peers are simply discovered
when sending their regular DNCP protocol TLVs using multicast.

When receiving a Node Endpoint TLV (Section 7.2.1) on an endpoint
from an unknown peer:
o If received over unicast, the remote node MUST be added as a peer
on the endpoint, and a Peer TLV (Section 7.3.1) MUST be created
for it.
o If received over multicast, the node MAY be sent a (possibly rate-
limited) unicast Request Network State TLV (Section 7.1.1).
If keep-alives specified in Section 6.1 are NOT sent by the peer
(either the DNCP profile does not specify the use of keep-alives or
the particular peer chooses not to send keep-alives), some other
existing local transport-specific means (such as Ethernet carrier
detection or TCP keep-alive) MUST be used to ensure its presence. If
the peer does not send keep-alives, and no means to verify presence
of the peer are available, the peer MUST be considered no longer
present, and it SHOULD NOT be added back as a peer until it starts
sending keep-alives again. When the peer is no longer present, the
Peer TLV and the local DNCP peer state MUST be removed. DNCP does
not define an explicit message or TLV for indicating the termination
of DNCP operation by the terminating node; however, a derived
protocol could specify an extension, if the need arises.
If the local endpoint is in the Multicast-Listen+Unicast transport
mode, a Peer TLV (Section 7.3.1) MUST NOT be published for the peers
not having the highest node identifier.
4.6. Data Liveliness Validation
Maintenance of the hash tree (Section 4.1) and thereby network state
hash updates depend on up-to-date information on bidirectional node
reachability derived from the contents of a topology graph. This
graph changes whenever nodes are added to or removed from the network
or when bidirectional connectivity between existing nodes is
established or lost. Therefore, the graph MUST be updated either
immediately or with a small delay shorter than the DNCP profile-
defined Trickle Imin whenever:
o A Peer TLV or a whole node is added or removed, or
o The origination time (in milliseconds) of some node's node data is
less than current time - 2^32 + 2^15.
The artificial upper limit for the origination time is used to
gracefully avoid overflows of the origination time and allow for the
node to republish its data as noted in Section 7.2.3.

The topology graph update starts with the local node marked as
reachable and all other nodes marked as unreachable. Other nodes are
then iteratively marked as reachable using the following algorithm: A
candidate not-yet-reachable node N with an endpoint NE is marked as
reachable if there is a reachable node R with an endpoint RE that
meets all of the following criteria:
o The origination time (in milliseconds) of R's node data is greater
than current time - 2^32 + 2^15.
o R publishes a Peer TLV with:
* Peer Node Identifier = N's node identifier
* Peer Endpoint Identifier = NE's endpoint identifier
* Endpoint Identifier = RE's endpoint identifier
o N publishes a Peer TLV with:
* Peer Node Identifier = R's node identifier
* Peer Endpoint Identifier = RE's endpoint identifier
* Endpoint Identifier = NE's endpoint identifier
The algorithm terminates when no more candidate nodes fulfilling
these criteria can be found.
DNCP nodes that have not been reachable in the most recent topology
graph traversal MUST NOT be used for calculation of the network state
hash, be provided to any applications that need to use the whole TLV
graph, or be provided to remote nodes. They MAY be forgotten
immediately after the topology graph traversal; however, it is
RECOMMENDED to keep them at least briefly to improve the speed of
DNCP network state convergence. This reduces the number of queries
needed to reconverge during both initial network convergence and when
a part of the network loses and regains bidirectional connectivity
within that time period.
5. Data Model
This section describes the local data structures a minimal
implementation might use. This section is provided only as a
convenience for the implementor. Some of the optional extensions
(Section 6) describe additional data requirements, and some optional
parts of the core protocol may also require more.

A DNCP node has:
o A data structure containing data about the most recently sent
Request Network State TLVs (Section 7.1.1). The simplest option
is keeping a timestamp of the most recent request (required to
fulfill reply rate limiting specified in Section 4.4).
A DNCP node has the following for every DNCP node in the DNCP
network:
o Node identifier: the unique identifier of the node. The length,
how it is produced, and how collisions are handled is up to the
DNCP profile.
o Node data: the set of TLV tuples published by that particular
node. As they are transmitted in a particular order (see Node
State TLV (Section 7.2.3) for details), maintaining the order
within the data structure here may be reasonable.
o Latest sequence number: the 32-bit sequence number that is
incremented any time the TLV set is published. The comparison
function used to compare them is described in Section 4.4.
o Origination time: the (estimated) time when the current TLV set
with the current sequence number was published. It is used to
populate the Milliseconds Since Origination field in a Node State
TLV (Section 7.2.3). Ideally, it also has millisecond accuracy.
Additionally, a DNCP node has a set of endpoints for which DNCP is
configured to be used. For each such endpoint, a node has:
o Endpoint identifier: the 32-bit opaque locally unique value
identifying the endpoint within a node. It SHOULD NOT be reused
immediately after an endpoint is disabled.
o Trickle instance: the endpoint's Trickle instance with parameters
I, T, and c (only on an endpoint in Multicast+Unicast transport
mode).
and one (or more) of the following:
o Interface: the assigned local network interface.
o Unicast address: the DNCP node it should connect with.
o Set of addresses: the DNCP nodes from which connections are
accepted.

For each remote (peer, endpoint) pair detected on a local endpoint, a
DNCP node has:
o Node identifier: the unique identifier of the peer.
o Endpoint identifier: the unique endpoint identifier used by the
peer.
o Peer address: the most recently used address of the peer
(authenticated and authorized, if security is enabled).
o Trickle instance: the particular peer's Trickle instance with
parameters I, T, and c (only on an endpoint in unicast mode, when
using an unreliable unicast transport).
6. Optional Extensions
This section specifies extensions to the core protocol that a DNCP
profile may specify to be used.
6.1. Keep-Alives
While DNCP provides mechanisms for discovery and adding new peers on
an endpoint (Section 4.5), as well as state change notifications,
another mechanism may be needed to get rid of old, no longer valid
peers if the transport or lower layers do not provide one as noted in
Section 4.6.
If keep-alives are not specified in the DNCP profile, the rest of
this subsection MUST be ignored.
A DNCP profile MAY specify either per-endpoint (sent using multicast
to all DNCP nodes connected to a multicast-enabled link) or per-peer
(sent using unicast to each peer individually) keep-alive support.
For every endpoint that a keep-alive is specified for in the DNCP
profile, the endpoint-specific keep-alive interval MUST be
maintained. By default, it is DNCP_KEEPALIVE_INTERVAL. If there is
a local value that is preferred for that for any reason
(configuration, energy conservation, media type, ...), it can be
substituted instead. If a non-default keep-alive interval is used on
any endpoint, a DNCP node MUST publish an appropriate Keep-Alive
Interval TLV(s) (Section 7.3.2) within its node data.

6.1.1. Data Model Additions
The following additions to the Data Model (Section 5) are needed to
support keep-alives:
For each configured endpoint that has per-endpoint keep-alives
enabled:
o Last sent: If a timestamp that indicates the last time a Network
State TLV (Section 7.2.2) was sent over that interface.
For each remote (peer, endpoint) pair detected on a local endpoint, a
DNCP node has:
o Last contact timestamp: A timestamp that indicates the last time a
consistent Network State TLV (Section 7.2.2) was received from the
peer over multicast or when anything was received over unicast.
Failing to update it for a certain amount of time as specified in
Section 6.1.5 results in the removal of the peer. When adding a
new peer, it is initialized to the current time.
o Last sent: If per-peer keep-alives are enabled, a timestamp that
indicates the last time a Network State TLV (Section 7.2.2) was
sent to that point-to-point peer. When adding a new peer, it is
initialized to the current time.
6.1.2. Per-Endpoint Periodic Keep-Alives
If per-endpoint keep-alives are enabled on an endpoint in
Multicast+Unicast transport mode, and if no traffic containing a
Network State TLV (Section 7.2.2) has been sent to a particular
endpoint within the endpoint-specific keep-alive interval, a Network
State TLV (Section 7.2.2) MUST be sent on that endpoint, and a new
Trickle interval started, as specified in step 2 of Section 4.2 of
[RFC6206]. The actual sending time SHOULD be further delayed by a
random time span in [0, Imin/2].
6.1.3. Per-Peer Periodic Keep-Alives
If per-peer keep-alives are enabled on a unicast-only endpoint, and
if no traffic containing a Network State TLV (Section 7.2.2) has been
sent to a particular peer within the endpoint-specific keep-alive
interval, a Network State TLV (Section 7.2.2) MUST be sent to the
peer, and a new Trickle interval started, as specified in step 2 of
Section 4.2 of [RFC6206].

6.1.4. Received TLV Processing Additions
If a TLV is received over unicast from the peer, the Last contact
timestamp for the peer MUST be updated.
On receipt of a Network State TLV (Section 7.2.2) that is consistent
with the locally calculated network state hash, the Last contact
timestamp for the peer MUST be updated in order to maintain it as a
peer.
6.1.5. Peer Removal
For every peer on every endpoint, the endpoint-specific keep-alive
interval must be calculated by looking for Keep-Alive Interval TLVs
(Section 7.3.2) published by the node, and if none exist, use the
default value of DNCP_KEEPALIVE_INTERVAL. If the peer's Last contact
timestamp has not been updated for at least a locally chosen
potentially endpoint-specific keep-alive multiplier (defaults to
DNCP_KEEPALIVE_MULTIPLIER) times the peer's endpoint-specific keep-
alive interval, the Peer TLV for that peer and the local DNCP peer
state MUST be removed.
6.2. Support for Dense Multicast-Enabled Links
This optimization is needed to avoid a state space explosion. Given
a large set of DNCP nodes publishing data on an endpoint that uses
multicast on a link, every node will add a Peer TLV (Section 7.3.1)
for each peer. While Trickle limits the amount of traffic on the
link in stable state to some extent, the total amount of data that is
added to and maintained in the DNCP network given N nodes on a
multicast-enabled link is O(N^2). Additionally, if per-peer keep-
alives are used, there will be O(N^2) keep-alives running on the link
if the liveliness of peers is not ensured using some other way (e.g.,
TCP connection lifetime, Layer 2 notification, or per-endpoint keep-
alive).
An upper bound for the number of peers that are allowed for a
particular type of link that an endpoint in Multicast+Unicast
transport mode is used on SHOULD be provided by a DNCP profile, but
it MAY also be chosen at runtime. The main consideration when
selecting a bound (if any) for a particular type of link should be
whether it supports multicast traffic and whether a too large number
of peers case is likely to happen during the use of that DNCP profile
on that particular type of link. If neither is likely, there is
little point specifying support for this for that particular link
type.

If a DNCP profile does not support this extension at all, the rest of
this subsection MUST be ignored. This is because when this extension
is used, the state within the DNCP network only contains a subset of
the full topology of the network. Therefore, every node must be
aware of the potential of it being used in a particular DNCP profile.
If the specified upper bound is exceeded for some endpoint in
Multicast+Unicast transport mode and if the node does not have the
highest node identifier on the link, it SHOULD treat the endpoint as
a unicast endpoint connected to the node that has the highest node
identifier detected on the link, therefore transitioning to
Multicast-listen+Unicast transport mode. See Section 4.2 for
implications on the specific endpoint behavior. The nodes in
Multicast-listen+Unicast transport mode MUST keep listening to
multicast traffic to both receive messages from the node(s) still in
Multicast+Unicast mode and react to nodes with a greater node
identifier appearing. If the highest node identifier present on the
link changes, the remote unicast address of the endpoints in
Multicast-Listen+Unicast transport mode MUST be changed. If the node
identifier of the local node is the highest one, the node MUST switch
back to, or stay in, Multicast+Unicast mode and form peer
relationships with all peers as specified in Section 4.5.